Addition polymerization reactions of vinyl monomers can involve anionic, free radical, or cationic intermediates. The reaction mechanism for polymerization is illustrated in the scheme below in which I represents an initiator; the asterisk signifies a negative charge, a free radical, or a positive charge; and Z represents hydrogen, halogen, or an organic group. ##STR1## Distinct initiation, propagation, and termination phases of a polymerization constitute a so-called chain reaction when the termination reaction provides an active by-product (designated I'* above) capable of initiating another polymerization sequence in addition to inactive polymer.
Whether polymerization involves anionic, free radical, or cationic intermediates is largely determined by the nature of Z in the vinyl monomer. Although many vinyl monomers are efficiently polymerized by free radical initiation, most polymerize ionically, if at all, by only one kind of active center. Methyl acrylate (in which Z.dbd.CO.sub.2 CH.sub.3), for example, polymerizes efficiently by radical and anionic initiation, but essentially not at all by cationic means. Generally speaking, Z groups which provide electronic and resonance stabilization by releasing electron density facilitate polymerization via cationic intermediates, while Z groups which withdraw electrons stabilize anionic intermediates.
That the azlactone group is electron withdrawing relative to hydrogen and 2-alkenyl azlactones (2-oxazolin-5-ones) possess 2-alkenyl groups which are electron deficient relative to ethylene are indicated by at least two factors. First of all, 2-vinyl-4,4-dimethylazlactone (VDM) can be effectively polymerized employing free radical techniques (cf. J. K.. Rasmussen, S. M. Heilmann, and L. R. Krepski, "Polyazlactones" in Encyclopedia of Polymer Science and Engineering, Volume 11, Second Edition, John Wiley & Sons, Inc.: New York, 1988, pp. 558-571). An indication of the electron supplying/withdrawing behavior of the azlactone group and the propensity of VDM to polymerize by ionic active centers can be obtained by examining free radical copolymerization with styrene. When this experiment was conducted in the above reference, the "e" value or measure of the polarity of the vinyl group in VDM was determined to be +0.65. By comparison with the e value for methyl acrylate of +0.64 (cf. R. Z. Greenley, "Q and e Values for Free Radical Copolymerization of Vinyl Monomers and Telogens" in Polymer Handbook edited by J. Brandrup and E. H. Immergut, Third Edition, John Wiley & Sons, Inc.: New York, 1989, II-267 to II-274), the azlactone heterocycle possesses approximately the same electron withdrawing capability as the carbomethoxy group and would be expected to stabilize anionic intermediates.
Another indication of the relative electron deficiency of the 2-alkenyl group in VDM can be obtained from its .sup.13 C-NMR spectrum. K. Hatada et al., Makromol. Chem., 178, 2413-2419 (1977) have utilized the relative position of the resonance of the 1-carbon of a terminal olefin to successfully predict whether the olefin will polymerize by anionic or cationic initiation. Using the carbon disulfide .sup.13 C resonance as a reference signal (193.7 ppm relative to the more traditional tetramethylsilane reference), these workers observed that monomers with 1-carbon resonances of about 100 ppm upfield, i.e., to the right or at higher energy, from the carbon disulfide resonance polymerized by cationic intermediates. Monomers with 1-carbons resonating at about 70 ppm responded to anionic initiation. Since VDM exhibits a 1-carbon resonance at 64.7 ppm on this scale, one would predict that 2-alkenyl azlactones would respond to anionic but not to cationic polymerization techniques.
Reports exist of electron deficient olefins which oligomerize or polymerize in the presence of acid. Tomalia et al. (Polymer J., 1980, 12, 661) motivated by the observation "that a variety of unidentified polymers, gels, or oligomeric syrups were readily formed by merely allowing 2-alkenyl-2-oxazolines to come in contact with Bronsted acids at room temperature" initially reported the oligomerization and polymerization of 2-isopropenyl-2-oxazoline (IPO). IPO is an electron deficient olefin as indicated by the e value of +0.34 for the 4,4-dimethyl derivative (Polymer Handbook, II-271) and a .sup.13 C-NMR 1-carbon resonance of 73 ppm upfield from the carbon disulfide resonance. They reported the formation of cyclic dimers in the presence of strong monoprotic Bronsted acids such as trifluoromethanesulfonic acid and low molecular weight (M.sub.N =900 to 2500) polymers when strong diprotic Bronsted acids such as sulfuric acid were utilized. These results are summarized in the Scheme below: ##STR2## With the exception of the six-membered ring dimer, all proposed structures involve the oxazoline nitrogen as a main-chain atom resulting from Michael or 1,4-addition of the nitrogen to the enone-like system of the 2-isopropenyl-2-oxazolinium species followed by proton transfer from nitrogen to carbon.
Similarly, Saegusa et al. (Polymer J., 1987, 19, 557) reported, based on the .sup.1 H-NMR spectrum, that 2-vinyloxazolinium fluorosulfonate, prepared from 2-vinyl-2-oxazoline and fluorosulfonic acid, provided a mixed polymer of predominantly monomer units in which nitrogen was present in the main-chain (80%) and a minor amount of vinyl polymerized units (20%). ##STR3##